Background It is still subject to controversy if an increased vertical opening (VO) is beneficial in oral appliance therapy for the treatment of obstructive sleep apnoea. Each oral appliance has a given thickness causing VO. Therefore, evaluation of the effects of the amount of VO on pharyngeal dimensions is mandatory. Methods The effects of VO on the cross-sectional area of the upper airway at the level of the tongue base during sleep endoscopy were scored and categorised. Results The figures demonstrate the possible effects of VO on pharyngeal collapse relative to the baseline cross-sectional area and the maximal comfortable protrusion of the mandible. Thirty-two patients (80%) showed an adverse effect of VO (Fig. 1), one patient (2.5%) had a positive effect (Fig. 2), and seven patients (17.5%) demonstrated an indifferent effect (Fig. 3). Conclusion Based on literature, the effect of VO on pharyngeal collapse is unclear and the therapeutic impact of VO is not determined. The results of the present study indicate that the effect of VO on the degree of pharyngeal collapse as assessed during sleep endoscopy tends to be adverse, causing an increase in collapsibility in the majority of patients.